219 



lorn and these effect digestion, eventually leaving the coelom, with 

 the egesta, through the nephridio-pores. 



Thus the coelom is the seat of monocytic digestive activity and 

 its fluid is a nutritive fluid containing all the nutritive products of mono- 

 cytic digestion and hence also it is not surprising to find that from 

 the walls of the coelom, in close contact with this fluid are evolved 

 the muscles and sexual cells. 



Outside the coelom is the space which represents the remains of 

 the haemocoele cavity ^^ but now no longer connected with nutrition 

 as no monocytic digestion takes place in it ; in fact, this cavity repre- 

 sents only the outer excretory zone of the haemocoele, as found in 

 the diploblastic form. 



Thus the body fluids in a typical coelomate form are divided in 

 two areas, separated from each other by the walls of the coelom. The 

 haemocoele cavity as here described, differentiates into the vascular 

 sy stemmo, which thus arises essentially as a system of vessels ful- 

 filling the function of carrying the waste products of metabolism (ex- 

 cretory and respiratory) to the outer layer. 



Thus in many of the lowest coelomata (such as the Polychaeta) 

 in which a vascular system apart from the coelom, occurs, the function 

 of the vascular fluid appears to be one of carrying waste products to 

 the excretory organs (skin, nephridia, branchiae) and for that purpose 

 the vascular system has branches from all the principal organs to the 

 nephridia and skin and to the branchiae. The fluid itself also has a 

 pigment diffused throughout it, either haemoglobin ^^ or some allied 

 compound. In Glycera, Phoronis and Capitella, the pigment is carried 

 by cells, whilst in other Polychaeta, Hirudinea, and Tuibellaria it is 

 diffuse. Haemocyanin appears to serve the same function in Mollusca 

 and Arthropoda^''. 



The blood in these forms is no doubt also partly nutritive but 

 probably so, only by virtue of absorbing the products of poly cy tic 

 digestion. 



In the diploblastic form, the nutritive results of poly cy tic digestion 

 diffuse through the endoderm, and simply re-inforce the nutritive fluid 



5* It will be noticed that in Hydra, thehydroids and the typical gastrula, the 

 'primary body-cavity' or 'haemocoele' has atrophied in correlation to the atrophy 

 of the physiological process of monocytic ingestive immigration. This is an additio- 

 nal reason for regarding these types as specialised from the primitive type as exempli- 

 fied by Medusae with haemocoelic mesogloea, and Sponges. 



55 Although a disputed point, a balance of evidence appears in favour of the 

 derivation of the vascular system from a specialised part of the segm. -cavity. See 

 Hertwigs, 0. and R. Embryology of Vertebrata. 



56 E. R. Lankester, Proc. Roy. Soc. Vol. XXI. 



67 C. A. McMunn, Q. J. M. S. XXV: with bibliography. 



13* 



